This invention relates to a separation device, and is particularly, although not exclusively, concerned with a separation device for separating components of an incoming mixture comprising liquid phases of different densities, and a solid component.
The separation of components and phases of a fluid mixture is important in many industrial processes. In the context of this specification, the term “phase” is used not simply to refer to different states (solid, liquid or gas) of a material, but more generally to embrace different materials that are in the same state as each other (e.g. two immiscible liquid components of different densities).
The separation of immiscible liquids is particularly challenging, especially when their densities differ only slightly. If solids are present in the mixture, the solids can provide nucleation sites in one or other of the liquid components, so promoting the formation of stable emulsions. If such stable emulsions form, they can be very difficult to break in order to enable separation of the two liquid phases of the emulsion.
While separation processes can be conducted in batches, it is generally preferred, in industrial processes, for separation to occur in a steady state continuous flow method.
Known processes for separating fluid mixtures can generally be regarded as operating either on a cyclone principle or a centrifuge principle. In cyclone separators, the flow is generally introduced into a chamber in a tangential manner at high energy, so inducing a rapidly rotating flow pattern within the chamber, causing lighter components to migrate towards the chamber axis, while heavier components migrate towards the outside. Although flow guides may be provided in cyclone separators, these tend to be fixed and it is desirable for the interior of the chambers to offer minimal obstruction to the circulating flow within them.
Typically, a centrifuge comprises a vessel which is rotatable about an axis. The vessel is rotated at high speed, and, again, denser components of a mixture migrate to the outside while lighter components accumulate nearer the centrifuge axis. In some cases, the outer wall of the vessel is porous, so that liquid components can be extracted, leaving solid materials accumulated on the porous wall.
Known separators, whether cyclones or centrifuges, suffer from a lack of flexibility. Consequently, they are unable to process incoming mixtures adequately at variable flow rates, or where the parameters of the incoming mixture change. For example, known separators are often unable to separate mixtures adequately where the ratio of different fluid components changes. In addition, solid components of incoming mixtures have a tendency to accumulate within the separators, which are unable to “self-clean”. Consequently, solids deposits build up over time, requiring the separator to be taken off-line for cleaning purposes.
Known separators often do not work adequately in a pressurized system, or require pumps to pressurize the fluids being processed.
In cyclone separators, the rotational velocity of the fluid within the separator is uncontrolled, and such changes in rotational velocity can upset the efficiency of separation of immiscible liquids. The requirement for centrifuges to rotate at high speed imposes a limitation on their size and capacity. Also, centrifuges do not have a low G zone where solids can concentrate without compacting.
The lack of control in cyclones and centrifuges makes it difficult or impossible to vary the residence time within the unit of individual components of the original mixture.
Current separators cannot easily be retro-fitted into existing processes as used, for example, in the oil and gas production industry to replace internal components on tail end production, where the liquids to be processed contain water. If the water is a continuous phase in the mixture, i.e. the mixture contains more water than oil, the deliverable may be oil with a reduced volumetric flow rate delivered to a further process for polishing to achieve the required oil quality, while the water may either be clean enough to be discharged into the environment or require tertiary treatment prior to discharge or reinjection into the production zone of the oil reservoir. In the case of heavy oil production, particularly when the oil is produced by pumping it out of the reservoir, the final treatment stage often has water in relatively small quantities, which needs to be separated from the heavy oil, as in processes for dehydrating crude, in which a counter-current flow of clean water is generated within the unit to desalt the crude.
The current technology also has inadequate performance in the treatment and removal of sulphur species (sweetening) from crude oil.
The following documents constitute examples of existing separators.
US 2003/0000144 discloses a gasification reactor apparatus in which solids materials are fed to a reactor vessel containing rotating paddles which direct the solid material to the outside of the vessel while produced gas is extracted from the centre. The material admitted to the vessel is not a mixture, but is the solid material only.
U.S. Pat. No. 4,702,837 discloses a rotary vortex separator comprising a vessel which is rotated about its longitudinal axis to achieve separation of a water/oil mixture.
EP 0226405 discloses a fuel pump arrangement in which an impeller concentrates air within the fuel towards the axis of the impeller so that fuel which is substantially free of air can be dispensed.
U.S. Pat. No. 5,207,810 discloses a downhole gas separator having a vaned rotor operating in an outer cylinder. Mixture is circulated by the rotor, and gas is separated from liquid components of the mixture under centrifugal force.
U.S. Pat. No. 5,271,163 discloses a treatment vessel for flowable materials in which a flowable material is acted upon by paddles mounted on a rotor while subjected to a flow of gas.
U.S. Pat. No. 5,630,557 discloses a grinding device in which fine material formed by the grinding process passes through a sifter rotor on the way to the outlet. The sifter rotor acts to prevent grinding beads from passing to the outlet.
According to the present invention there is provided a separation device comprising an outer wall defining a separation chamber in which a motor-driven rotor is disposed for rotation relative to the outer wall about a rotor axis, the separation chamber having an inlet for a mixture to be separated, and a plurality of outlets at different distances from the rotor axis for discharging respective phases of the mixture from the separation chamber, the outlets including two fluid phase outlets, comprising a lighter phase outlet and a heavier phase outlet, the heavier phase outlet being situated radially outwardly of the lighter phase outlet with respect to the rotor axis, the rotor having at least one vane extending outwardly with respect to the rotor axis, and being surrounded at least partially by a perforated screen which is fixed with respect to the outer wall and is spaced from the outer wall to define an annular zone between the screen and the outer wall, the outlets further comprising a solids outlet provided in the outer wall.
In an embodiment in accordance with the present invention, the outer wall is cylindrical and the rotor is coaxial with the outer wall. The vane may be one of a plurality of vanes, and the vanes may extend substantially radially with respect to the rotor axis. At the outer periphery of the rotor, at least one of the vanes may have a tip region which is inclined to the radial direction.
The vanes may extend axially with respect to the rotor axis, and may occupy substantially the full axial extent of the separation chamber. The vanes may be mounted on a shaft, which may be adapted for driving connection to a motor for driving the rotor. The rotor may be provided with support plates, for example in the form of radial discs. The support plates may have apertures to enable flow to take place from one side of each support plate to the other. The apertures may also be adapted to receive the vanes.
The inlet to the separation chamber may be disposed tangentially with respect to the axis of rotation of the rotor, so as to induce a rotary motion within the separation chamber. The separation chamber may comprise a central region occupied by the rotor, and end region at opposite ends of the central region. With such a construction, the inlet may open into one of the end regions of the chambers, and the outlet may open into the other end region of the chamber.
The screen may extend along substantially the full length of the rotor.
The solids outlet may comprise a plurality of outlet ports distributed lengthwise of the outer wall. The solids outlet is preferably disposed in the lower region of the outer wall, and may be connected to a fluidizing vessel to enable transport of particulate solids materials drawn from the separator.
The lighter phase outlet may comprise an annular slot centered on the rotor axis, opening into a lighter phase compartment. The lighter phase compartment may have a settlement zone, and swirl reducing stator elements may be disposed within the lighter phase compartment between the annular slot and the settlement zone. A gas outlet may be provided, which communicates with the settlement zone to enable gas to be vented from the lighter phase compartment. A lighter phase outlet may open into the lighter phase compartment, for example in a tangential direction with respect to the rotor axis. If swirl reducing stator elements are provided, so that there is a relatively quiescent settlement zone within the lighter phase compartment, the gas outlet may open into the lighter phase compartment at an upper region of the settlement zone, and the lighter phase outlet may open into the lighter phase compartment at a lower region of the settlement zone. However, if the swirl reducing stator elements are not present, or effect only a limited reduction in swirl in the lighter phase compartment, the gas outlet may be situated radially inwardly of the lighter phase outlet with respect to the rotor axis.
The heavier phase outlet may comprise an annular slot centered on the rotor axis, opening into a heavier phase compartment into which the heavier phase outlet opens. The heavier phase outlet may extend tangentially with respect to the rotor axis.
Control means may be provided for controlling at least one operating parameter of the separation device. The parameters which may be controlled may be one or more of the rotational speed of the rotor, the temperature within the separation chamber, and the outlet pressure of at least one of the outlets.
The control means may be responsive to at least one process parameter. This process parameter may be one or more of the volume flow rate of the mixture through the inlet, the ratio of phases in the inflowing mixture, the radial position of an interface between the phases in the separator, for example an interface between heavier and lighter liquid phases, the density, temperature or viscosity of the inflowing mixture, the viscosity of at least one of the phases, and a parameter of the solid content in the mixture, for example the particle size, shape factor or concentration.
The separation device may also have an outlet for removal of a rag layer.
The separation device may comprise part of an extraction unit comprising a vessel which receives a mixture to be separated, and in which a separation device as defined above is situated.
For a better understanding of the present invention and to show more clearly how it may be carried in effect, reference will now be made, by way of example, to the accompanying drawings, in which